This proposal is based on our studies which have shown that action of trypsin or thrombin at the cell surface is sufficient to cause nonproliferating chick embryo cells to divide. Our general objective now is to define the cell surface events involved in this initiation. We will study the role of a 45,000 dalton cell surface component (45k) that we have found is removed by thrombin in cells that are responsive to its mitogenic action, but which is thrombin-resistant in four populations of cells we have isolated that divide after serum treatment but not after thrombin treatment. We will test a hypothesis, consistent with our current data, that the duration of removal of 45k determines the increase in cell number when a nonproliferating cell population is treated with thrombin. To examine the generality of removal of 45k in cells that divide after thrombin treatment, and resistance of 45k in cells that are unresponsive to the mitogenic action of thrombin, we will conduct studies on additional kinds of cloned responsive and unresponsive cells. As we develop better procedures to label and separate cell surface proteins, we will search for other surface components that are cleaved by thrombin in responsive cells but are absent or thrombin-resistant in cells that are unresponsive to the mitogenic action of thrombin. Our recent studies have led to the identification of a single affinity class of cell surface receptor for thrombin. Studies on 125I-thrombin binding and increase in cell number as a function of thrombin concentration indicate a relationship between binding and initiation. We now plan to evalute the role of the thrombin receptor in initiation of cell division by thrombin, and to probe the relationship between the receptor and the thrombin-sensitive cell surface components that appear to be involved in initiation by thrombin. Clones which lack a functional thrombin receptor will play a key role in these studies. We have planned collaborative experiments with Dr. C. Fred Fox to label the thrombin receptor and determine its molecular weight, using a photoaffinity crosslinking derivative of 125I-thrombin.